In court papers filed in New York, the YSL suit brings in some historical big guns:

"Red outsoles are a commonly-used ornamental design feature in footwear, dating as far back as the red shoes worn by King Louis XIV in the 1600s and the ruby red shoes that carried Dorothy home in The Wizard of Oz."

Furthermore, YSL is making the argument that Louboutin had full knowledge of such fashion precendents when the brand seized the rights to the red sole and made statements such as "No one before me has ever used a coloured sole to define a brand's identity."